Originally Posted by The Weapon

Just FYI:

From the Bose HU you have 4 channels (Low Level) going to Bose AMP
Bose Amp goes to 6 Speaker Outs (2 Doors, 2 tweets, 2 rear) and another low level sub channel to the sub amp. Sub Amp goes to 2 Speaker Outputs to Subs.

For mids/High full range signal:
Connecting before the Bose will give you massive engine noise. I've tried it several different ways. From Bose HU to Bose AMP you have - / + / - wires. The 2nd Negative cancels out noise when its processed through the AMP. Once you disconnect it introduces some nasty noise. The solution i found was connection from Bose AMP Speaker Outputs and summing and flatten, and re-adjust. No Engine noise and sounds very clear.

If you want a sub only:
You can connect from blue and violet wire that comes from the bose amp to Sub Amp but you'll notice that extra (-) again. You can't hear the engine noise through the sub so its not really a big deal but I also found the signal strength is much better connecting from Sub Amp Speaker Output as well instead of Low Level frequency.
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Another option I've been thinking of is trying to to Keep the Bose Amp connected while splitting off the signal rather than completely disconnecting it. It will continue to cancel out the engine noise with the -/+/- set up it has but i don't know if the new amp would just introduce noise anyways. I haven't tried it yet because Sound Processor only accepts High Speaker Level inputs.